For some time, studies have emphasized the importance of considering the effects of impulse noise coupled by crosstalk between cable pairs, especially near end crosstalk (NEXT), as a predominant source of error in digital transmission systems. Near end crosstalk is that crosstalk which occurs at the same end of a multipair cable as a disturbance. See Cravis, H. and Crater, T. V, "Engineering of T1 Carrier System Repeater Lines". B.S.T.J. 42, No. 1, March 1963, pp. 431-86.
This is particularly true in a system which is referred to as a T1 system in which voice and data conductor pairs are provided in a single cable and enclosed in a common sheath system. In a T1 system, whereas a single pair of electrical conductors may be used for voice transmission, it is common to use two pairs for data, one for receiving data signals from a remote terminal and one for transmitting data signals to the remote terminal.
A T1 system is one which uses a 1.5 megabit per second signal and, more particularly, a 1.5 megabit alternate mark inversion (AMI) signal. AMI is a coding scheme whereby successive consecutive pulses must be of opposite polarity. In such a system, a "0" is transmitted with the absence of a mark. A mark is transmitted as a "1" or high voltage pulse having a magnitude of 3 volts and either a positive or negative polarity. However, the next successive mark would have the opposite polarity. A bipolar violation occurs when two consecutive high voltage pulses of the same polarity are transmitted. Generally, this condition is recognied by T1 receiving apparatus.
Impulse noise can have an adverse effect on the transmission of data signals in a T1 system. Relays, switches, rectifier power supplies, AC power wiring, test tones and ringing generators in central offices and remote terminals all have been identified as impulse noise sources. Impulses, though occurring spasmodically, typically are 20 to 40 dB stronger than any desired signal being transmitted and include many high frequency components. As a result, impulses are difficult to eliminate.
Cravis and Crater, op cit, recognized crosstalk interference from impulse noise as a problem for T1 transmission. Their solutions are twofold. First, they reduced the distance (spacing) between regenerations whenever close to sources of impulse noise. Typically, the reduction was 50%. This meant that the received signal, instead of being attenuated 30 to 35 dB, was attenuated only about 15 dB. The reduced spacing results in 15 to 20 dB stronger received signals, which are more resistant to interference. Second, they used some pair groups entirely for T1 and other pair groups entirely for voice. Keeping T1 and voice in separate pair groups reduces the impulse crosstalk interference. However, there is considerable evidence that most errors in T1 systems emanate from central offices.
These solutions, though far from perfect, are still used. They are not onerous for T1 usage as a trunk facility connecting central offices, nor are they onerous for T1 usage for loop carrier systems because, typically, trunk cables and loop-carrier cables extend several miles and include several repeater sections. Therefore, short sections near one or both ends are simple, expedient and not too expensive. Since the cables emanating from central offices usually have hundreds of pairs, dedicating certain pair groups is not inefficient. In the case of loop carrier, the T1 transmission ends at a remote terminal, usually still in a fair size cable so that using a pair group strictly for T1 is feasible. Subscriber stations are hundreds or thousands of feet away.
Today T1 carrier service is being delivered to subscribers. For example, in 1985 upwards of about 2000 such T1 communications lines were installed and it has been found that troublesome inpulse noise can be generated by certain telephone set operations.
When the ringing signal in a voice pair of conductors is interrupted, the disturbance can be at least 20 dB stronger than intersystem near end crosstalk. While such strong impulses are infrequent, they may be of concern, particularly as T1 service to customer premises continues to grow. Shortening of the end sections was tolerable when most systems extended several miles between central offices. Today T1 signals are being delivered to individual subscribers, over 50% of which are located less than 2.5 miles from central offices.
Station-generated noise on a voice conductor pair can cause errors in T1 data signals transmitted on another conductor pair enclosed in the same cable sheath system. In a typical optical communication distribution system, cables which include hundreds or thousands of pairs are routed from a central point, are branched into smaller cables, and as they near the subscribers, may have 25 pairs, for example. Within a premises, the cables may include four or only two pairs of conductors. In this situation, the voice pair is designated as the disturbing pair and the data pair, the disturbed pair. It has been found that the ringing of station apparatus can coexist with substantially error-free data transmission in a T1 system; however, upon pickup at the station apparatus, an undesirable voltage step is created which may adversely affect the data signal that has been transmitted. The station apparatus may be any which is capable of generating an abrupt, transient voltage. Voltage on one conductor pair shows up on another pair because of uncompensated mutual capacitance and inductance between pairs. Although cables typically are designed so as to have very little net coupling between conductor pairs, small residual capacitance and inductance unbalances remain and provide the mechanism for crosstalk coupling between the pairs.
When conductor pairs are coupled, the mechanism is in place for imparting a voltage impulse created on one pair when, for example, a telephone set on another line in the same cable goes off-hook. It is assumed that the telephone line is of the 48 volt common battery type and therefore would have a 48 volt DC charge. When the receiver of the telephone set is lifted from the switchhook, the connected cable pair is discharged suddenly, creating a voltage step. This sudden change in voltage is coupled through the mechanism of crosstalk to a closely coupled data conductor pair as impulse noise. If the receiver is lifted while the telephone set is being rung, the step voltage can be more than three times the normal line voltage. Another impulse generator is a rotary dial which repeatedly shorts the pair.
Various schemes have been available for dealing with the noise problem in analog systems. For example, diode limiters cause any disturbances on a line above a certain magnitude to be cut off. This problem also may be avoided by assigning voice and data to conductor pairs which are not closely coupled. However, this last-mentioned technique for avoiding impulse noise, which is called pair selection, is subject to human error, is of limited effect in cables having only a few pairs and is not commonly used.
Also well known are various schemes for separating the usable bandwidth of a single cable pair into separate channels. One method of doing this would include the step of dividing the usable bandwidth into frequency bands. Each band then could be used to transmit its own signal which could be analog or digital. Interference between the signals being transmitted in different frequency bands is properly termed crosstalk and, in such cases, is crosstalk between channels on the same pair.
The prior art also includes devices for dealing with impulse noise generators. As far as is known, devices for resisting the effect of impulse noise generators commonly are included in the circuit which is the target of the disturbance and which is referred to as the disturbed circuit. This may not always be a practical arrangement.
What is needed and what is not provided in the prior art are methods and a system for providing voice on one pair and substantially error-free transmitted T1 data signals on another pair, both pairs being disposed within the same cable. Desirably, apparatus for providing such error-free transmission is simplistic in design and can be included readily in existing systems and hopefully in the disturbing, rather than in the disturbed circuit.